1. Field of the Invention
This invention relates to an apparatus and process for producing strong degradation-resistant agglomerates or pellets of mineral ore and, more specifically, to such an apparatus and process which includes passing the agglomerates or pellets through an oxidation zone on a traveling grate prior to being discharged into a kiln having a reducing atmosphere. Once in the kiln, the agglomerates or pellets are first advanced through an induration zone of the kiln in order to provide sufficient strength to the agglomerates or pellets passing therethrough to enable them to better withstand stress in a reduction zone of the kiln in order to improve the physical characteristics of the pellets and, therefore, the yield.
2. Description of the Prior Art
U.S. Pat. No. 3,753,682 is directed to a ported rotary kiln process for direct reduction of oxides and sulfides of metallic materials to increase the metallic content thereof. In particular, the apparatus and process disclosed therein is for pre-reduction of mineral ore including a controlled admission of hydrocarbon fuel and oxidizing gases into a rotary kiln for an oxygen reduction treatment which is not intended to produce liquid metal but is intended to increase the metallic content of a particle, pellet or agglomerate in a generally solid state by reducing the oxygen content thereof. The oxygen content is reduced in order to provide a pretreated charge material having increased utility in chemical processes requiring a mineral in metallic form or for increasing the capacity and fuel efficiency of subsequent smelting and refining processes.
Although the process is primarily used, as suggested above, for metallic materials, those skilled in the materials processing art will recognize that other material may include compounds of elements which are not metallic that can be reduced in a similar manner. Accordingly, references hereinbelow to the reduction of metallic material or to the increase of metal content are equally applicable to other material including compounds of elements which are not actually a metal in order to increase the content of at least one of the elements thereof.
In the preferred process taught in U.S. Pat. No. 3,753,682, finely divided iron ore and coal along with bentonite are mixed and rolled into suitable agglomerates or particles by a balling drum which may be of the type shown in U.S. Pat. Nos. 1,994,718 and 2,411,873. The resulting properly-sized green balls, agglomerates or pellets are deposited on the feed end of an oxidizing traveling grate in the oxidizing atmosphere of an oxidation zone. In the oxidation zone, fully oxidized, afterburner exhaust gas from the reduction process is utilized to dry and preheat the green balls or pellets on the grate in successive process regions. Upon leaving an initial drying region of the oxidation zone, the balls may be fully dry but may not be strong enough to survive the transfer into the kiln which has a reduction zone including a reducing atmosphere for reducing the balls or pellets. The traveling grate carries the dry balls or agglomerates or pellets into a preheat region of a higher temperature than the drying region. The pellets or agglomerates can be partially oxidized by their passage through the preheat region and, in turn, can be strengthened for the grate-to-kiln transition and early stages of reduction.
The preheated pellets or agglomerates from the oxidation zone are transferred via an enclosed chute from the grate to the reduction zone in a rotary kiln. The prior art rotary kiln is inclined downwardly at a slight angle below horizontal so that, upon rotation, the pellets in a bed at the upper, inlet end of the kiln will tumble and mix as they advance through the kiln. The kiln has axial burners and peripheral ports and may be of a type described in U.S. Pat. No. 3,182,980. With the kiln being disposed at a slight downward angle, the pellets or agglomerates can tumble through the kiln to a chute for discharge to a cooler. Gas flow through the kiln is in the opposite direction of the movement of the pellets or agglomerates therethrough. The gas flow preferably results from fuel and air alternatively being injected into the kiln respectively through the peripheral ports located around and along the length of the kiln. Fuel is only injected into the bed of pellets in the kiln while air is only injected above the bed of pellets. In U.S. Pat. No. 3,753,682, the preferred reducing agent is natural gas which is primarily introduced into the kiln through the peripheral ports located below the bed of the pellets or agglomerates. As the pellets or agglomerates move axially through the kiln, the fuel ports in the periphery of the kiln pass beneath the bed of pellets or agglomerates. Natural gas, substantially free of any oxidizing gases, is passed through the ports and into the bed to make initial and intimate contact with the pellets or agglomerates which are at a temperature of about 2000 degrees Fahrenheit. The contact of such gases with the pellets or agglomerates in an oxide form at such a high temperature causes a portion of the gas to quickly pyrolytically decompose with an amount thereof being reformed as a strong reducing agent including, for example, carbon monoxide and hydrogen. At the same time, air is introduced into the peripheral ports above the bed in order to support combustion of the non-oxidized products of reduction that issue from the charge. This combustion provides heat to sustain operating temperature in the kiln and, because of the remaining chemical energy in exhaust gases therefrom, furnishes the necessary fuel for the afterburner and subsequent heat for the oxidation on the traveling grate in the oxidation zone.
After reduction in the kiln, the pellets or agglomerates which contain significantly increased metallic fractions are directed to a cooler. After being cooled, the resulting material is screened, sized and separated for determining what portion of the material is properly processed and what portion must be discarded or recycled.
Another apparatus disclosed in U.S. Pat. No. 3,753,682 uses prehardened agglomerates or particles which are provided to a hopper and then fed through a hood into the upper end of a ported rotary kiln. The kiln is provided with an unported preheat section followed by the reduction zone thereof which is provided with the peripheral ports for the introduction of natural gas and air. The kiln may be of the type described in U.S. Pat. No. 3,182,980 and the contents thereof may be discharged to an indirect cooler which may be of the type shown in U.S. Pat. No. 2,792,298. The pellets or agglomerates may be previously heat hardened by such systems as are disclosed in U.S. Pat. Nos. 2,750,272: 2,750,273: 2,750,274 or 2,925,336.
It is also suggested in U.S. Pat. No. 3,753,682 that either of the embodiments disclosed therein may incorporate an addition of coal with the ore as the ore leaves the oxidation zone of the traveling grate before the pellets are reduced in the reduction zone of the kiln. Additional and more detailed features of the preferred ported rotary kiln system are disclosed in "The Direct Reduction of Iron Ores With The ACCAR SYSTEM" and "Five Hundred Tonne Per Day ACCAR Direct Reduction Plant at Orissa, India" which were published by the Allis-Chalmers Corporation. ACCAR and ACCAR SYSTEM were trademarks of the Allis-Chalmers Corporation and are presently the trademarks of Boliden Allis, Incorporated. The systems disclosed in these publications can employ gases, liquid or solid fuels singly or in combination to obtain the desired reduction of iron ores. In both systems, coal is preferably fed to the inlet of the rotary kiln with the ore charge. Additionally, these publications include an explanation of the use of a coal slinger at the outlet end of the rotary kiln for the introduction of a quantity of coal to the interior thereof. Still further, additional details are provided to explain the screening, magnetic separation of the unburned coal (char) and reduced pellets, and distribution of the material after it is discharged from the cooler.
"GRATE-CAR Direct Reduction System From Allis-Chalmers" and "GRATE-CAR System For Pyro-processing Ferrous And Other Ores With Solid And/Or Fluid Fuels" were published by Allis-Chalmers Corporation to disclose an improved overall system for the direct reduction for material therein. GRATE-CAR was a trademark of Allis-Chalmers Corporation and is presently the trademark of Boliden Allis, Incorporated. As discussed in these publications, direct reduction feed stock traditionally included lump ore and/or iron oxide pellets which were fed at ambient temperature to a reduction vessel. This required that part of the vessel, whether a rotary or shaft kiln, be used as a preheater to elevate the feed stock to reaction temperature. As explained therein, conventional iron ore pelletizing and reduction processes employed two separate plants. In the pelletizing plant, the pelletized concentrate was heat treated and cooled. The pellets at ambient temperature were then fed to a reduction plant. With the development of the GRATE-CAR system, the pelletizing plant cooler was eliminated to result in a capital cost saving and a simplified process flow sheet. The charging of hot pellets to a rotary kiln also resulted in an energy savings. Still further, a smaller rotary kiln could be employed because additional kiln volume for pellet preheating would no longer appear to be required.
Generally, the GRATE-CAR system disclosed in the aforementioned publications consists of a traveling grate, a ported or nonported rotary kiln and a cooler connected in series. Pelletized concentrate is fed to the oxidizing traveling grate for drying and preheating in order to provide strength to the pellets. The hot preheated pellets are then fed directly into the rotary kiln. Solid fuel such as coal or other processing agents can be added to the grate discharge along with the pellets to the rotary kiln. In the case of a ported kiln, natural gas, oil or other fluid fuels can be introduced through the ports. Pellets discharging from the lower end of the kiln are cooled in a cooler.
One aspect of the GRATE-CAR system employs a kiln firing hood at the lower, discharge end of the rotary kiln which is designed to include a coal slinger. The coal slinger includes a pneumatic coal injection system which delivers coal to the downhill one-third or one-half of the kiln. The purpose of injecting the coal in this manner is to supplement the coal fed with the preheated pellets and to maximize the use of coal volatiles in the kiln. The reduced product and char discharge from the kiln into a rotary cooler where the material is cooled to about fifty degrees centigrade. The main control factors in the process are kiln temperature, exhaust gas composition and product quality. The optimum reducing condition for the kiln is maintained by adjusting the quantity of fuel and air inputs until a balance is achieved between the solids retention time in the kiln, temperature and product quality.
A specific installation which serves as an example of the GRATE-CAR system is described and disclosed in a publication entitled "Ilmenite Direct Reduction Project In Norway Using The GRATE-CAR Process" and published by Boliden Allis, Inc.
It should be recognized that some particular ores may be capable of being reduced by the prior art processes without producing any unacceptable fines which must be discarded or reprocessed. However, for many such ores, throughout the reduction processes described hereinabove, there is continuing concern regarding the strength and integrity of the pellets as they are passing through the oxidizing zone and into the reduction zone. Generally, throughout the reduction processes discussed hereinabove, there are various temperature, porosity and chemical changes to such green balls, pellets, or agglomerates which significantly affect their overall physical characteristics such as strength and subsequent ability to be properly reduced in order to produce a product of high metal content. Although the reduction in the systems described above is preferably provided in a ported or non-ported rotary kiln which is downwardly inclined to advance the pellets therethrough, the use of other kiln systems with other means for advancing the pellets would also depend on the integrity of the pellets for proper reduction. Accordingly, there remains a need for any apparatus or process which can be employed to further improve the overall physical characteristics, such as the overall strength, of the pellets to further insure their proper integrity as they are reduced in a kiln and subsequently discharged in a form having a higher metal content.
The above listed patents and publications are incorporated by reference as if the entire contents thereof were fully set forth herein.